Genetic testing of common carp in cages. 2. Influence of variation in initial weight on weight gain

Abstract Three tests are described, in which common carp were stocked into net cages. A positive association was found between weight gain and initial weight, both for individual carp and means of four to five fish. This association cannot be due to direct interactions among the groups, since different test groups were stocked separately. In the last test, seven genetic groups were stocked into separate cages. Their observed weight gains were corrected for the biassing effect of variation in initial weight, with the aid of multiply-nursed samples. This resulted in a reduction of the standard deviation of weight gain, a reduction in the association between initial weight and weight gain, and a change of ranking among the genetic groups tested. We conclude that reliable estimation of genetic differences in growth requires amending observed weight gains for variation in initial weight in separate testing cages. This has been previously demonstrated for communal pond testing.